Test: Thyristor Chopper Circuits

Current Limit Control of Chopper (CLC):
In this strategy, the chopper is turned on when the output current i₀ equals a preset value I₂.
The chopper is kept on till i₀ increases to another preset value I₁.
The chopper is turned off when i₀ equals I₁ and is kept off till the current i₀ decays to I₂.
The current i₀ is thus limited to a maximum of I₁ and a minimum of I₂ as shown in Figure.

The chopping frequency and the pulse width are dependent on the load parameters.
The advantage of current limit control is that the ripple (I₁ - I₂), in the load current i₀ can be adjusted to a small, predetermined value for a wide range of loads.
Application:
Given,
The maximum possible value of the accelerating current (I₂) = 300 A
Current pulsation or current ripple (ΔI) = 140 A
The waveform can be drawn as,

From the waveform,
The maximum limit of current pulsation (I₂) = I₁ + ΔI = 300 + 140 = 440 A

Concept:
In a LC circuit, the current flows in the circuit is given by,

Where, V_S­ is supply voltage in V
C is capacitance in F
L is inductance in H
ω₀ is resonant frequency in rad/sec

Calculation:
From the given circuit, V_S = 100 V
C = 1 μF
L = 20 mH

In voltage commutation circuit
Circuit turn off time
  for highly inductive load
t_m = RC ln 2 ------- for resistive load
∴ t_m = 15 × 8 × 10^-6 ln 2 = 83.18 μs

Concept:
Mode 1: T₁ will remain ON and conduct to load and diode T_A will remain OFF
Mode 2: T_A will be turned ON; the capacitor current will now flow in reverse direction & T1 stops conducting.
Therefore, the effective time period of the chopper is

Commutation time: It is the time taken to disconnect the load from the supply after the main thyristor is turned OFF.
Calculation:
Effective on period 

= (0.8 + 0.42) × 10^-3
T_ON’ = 1.22 ms

The size of capacitor enquired for commutation is